Vehicles often need to obtain power (e.g., 110-volt or 220-volt AC, or sometimes 12 or 24-volt DC) from a land-based supply (e.g., by connection to the electrical utility power grid). Some conventional vehicles include a conventional 110-volt power cord running from the vehicle and having a conventional three-prong NEMA-standard plug on the end (National Electrical Manufacturers Association), which plug is inserted into a residential-type 110-volt outlet. Such a configuration has proved unsuitable or lacking for high-current needs, in that the plug can come unplugged or can have too little contact force, resulting in arc faults (where intermittent high-resistance contact is made, and where an arc across a small gap will form), which have a possible fire hazard for the vehicle and or docking facility.
Residential electrical circuitry originally used a “two-pole, two-wire, two-prong” configuration with each receptacle having a hot slot (also called the phase, line, or hot slot), and a neutral/ground slot. These receptacles did not have a separate equipment-grounding mechanism or connection. One pole is called the hot, phase, line, or hot wire, and the other pole is called the neutral. In the two-pole configuration, the neutral also served as a ground. A receptacle is a device with recessed male or female contacts that is part of an outlet typically installed in a wall or on equipment, and which is intended to establish electrical connection with, and provide power to, an inserted plug. A wall-mounted duplex outlet will have two receptacles. A plug is a device with male blades which, when inserted into a receptacle, establishes connection between the conductors of the attached flexible cord and the conductors connected to the receptacle. With the original “two-pole, two-wire” scheme, the only grounding point was at the service entrance, where the neutral (white) conductor was grounded. At some point, the NEMA (National Electrical Manufacturers Association) configuration 1-15R required that the receptacle slot for the neutral wire (typically having white-colored insulation) be longer than the slot for the hot wire (typically having black- or red-colored insulation), and that the blade of the neutral wire on the plug be wider than the hot blade, in order that it could not be inserted into the shorter hot slot. This enables certain types of equipment, like power-supply transformers and home appliances, to have their external metal parts or casing grounded through the white neutral wire connection. Such equipment uses a polarized plug where the neutral plug blade is wider than the hot plug blade, ensuring that it can only be inserted into a NEMA 1-15R configuration receptacle with the correct orientation.
Many modern residential and industrial power outlets and power plugs now have what is termed a two-pole, three-wire, three-prong design, which in the U.S. is typically used for conventional 120 V.A.C. (volts alternating current) convenience power outlets. Such power outlets typically include two receptacles and are known as duplex outlets. These configurations provide a separate ground wire from the receptacle that is typically connected to neutral and ground/earth at the residential circuit-breaker box. A modern three-prong power plug has three male blades or prongs that are typically nickel plated, tin, or brass, and that are inserted into three respective female slots or sockets of a wall receptacle. The prongs of the power plug and the female slots or sockets of the wall-mounted power receptacle vary in terms of size and shape based upon the purpose that they serve. One of the prongs (the “ground prong”) is typically longer than the other two prongs, and in some embodiments is circular, semi-circular, or rounded in shape. Another of the prongs (the “neutral prong”) has a blade that is slightly wider than the third prong's blade (the “hot prong”). Many power plugs are still made with only the hot and neutral prongs (“two-prong power plugs”), and omit the ground prong. Such two-prong plugs are often polarized, with the neutral blade wider than the hot blade. A three-socket power receptacle will accept either two-prong or three-prong power plugs.
Corresponding to the three male prongs of the plug are three female slots or sockets (i.e., the hot socket, neutral socket, and ground socket) of the power receptacle into which the plug's prongs are inserted. The power receptacle's sockets are designed to accommodate the size and length variations and allow either two-prong or three-prong power plugs to be inserted, while preventing or making it difficult to insert a two-prong plug the wrong way (e.g., with the neutral prong of the plug inserted into the hot socket of the power receptacle). The neutral socket of the power receptacle and the neutral prong of the plug are wider than the hot socket that accepts the hot prong, such that the neutral plug is too wide to be inserted into the hot socket. As an additional safety feature, the ground prong of the plug is typically made longer than either the hot prong or the neutral prong, in order that it makes contact with the power receptacle first. Correspondingly, the ground socket that accepts the ground prong is deeper than the other two sockets so as to accommodate the increased length of the ground prong.
One reason for the three-prong design, and in particular the use of a ground prong, is to provide an electrical ground that can be connected to the outside of a device, or its metal frame or chassis, such that a person who is standing on or otherwise connected to ground will not get a shock from the device if the hot power voltage or a portion thereof is connected to the device frame by accident damage, aged components, insulation degradation, impact, or wiring mistake. If the person and the outside of the device are both at a ground voltage, there will be no current flow when the person is touching the outside of the device.
Another reason for the three-prong design relates to the need to dissipate and/or direct ambient and non-ambient electrical charges. A system of interconnected electrical circuits, such as those found in the typical residential house, acts like a capacitive antenna that can either build up and/or conduct ambient and non-ambient electrical power found in the atmosphere. For example, when a house is struck by lightening, absent the use of various ground prongs, the electrical energy of the lightening could be routed through all the ungrounded electrical circuits including appliances connected to these circuits. This electrical energy would destroy many of these ungrounded appliances. One solution to this problem is to provide a ground path to allow this electricity to be dissipated into the earth or ground.
Yet another reason behind the three-prong design, when mounted with the ground socket uppermost, may be to lessen the likelihood that a circuit could be formed directly across the hot and neutral prongs. Namely, the ground prong can act as a barrier or guard that prevents a piece of conductive material (for example, a cookie sheet) from slipping into the space between the power plug and power receptacle and forming a short circuit between the hot and neutral prongs. Were such a short circuit to occur, the high current can vaporize the metal prongs, which could cause a fire or other damage.
Power receptacles are typically set in a dual- or duplex-outlet configuration whereby two power receptacles are stacked one on top of one another. In most of these duplex-outlet power-receptacle configurations, the power-receptacle sockets are arranged such that the hot, neutral, and ground sockets have the same orientation, and wherein each feature of the upper receptacle is approximately 39 millimeters above the corresponding feature of the lower receptacle. Further, typically, the screw connectors for the neutral and ground wires are all on one side of the outlet device, and the screw connectors for the hot wire(s) are on the opposite side of the device. Further still, many companies and electrical inspectors recommend that conventional duplex outlets be installed having the hot and neutral slots, which are set parallel to each other, oriented vertically, with the hot slot on the left and the neutral slot on the right, and the ground socket of each receptacle set above these parallel slots, in what is called a ground-up orientation or configuration. Some electricians and homeowners prefer to have the ground socket below the hot and neutral sockets (with the hot slots on the right and the neutral slots on the left), in what is called a ground-down orientation or configuration.
One problem that occurs with such conventional residential plugs and sockets is that sliding and compressive forces must be overcome when inserting and withdrawing such plugs from their sockets. As current requirements increase (particularly for 50 amperes and above), it is quite difficult for a person of ordinary strength to insert or withdraw the plug, or, on the other hand, the spring force becomes weakened after repeated uses and the contacts are electrically unreliable and the resulting high-resistance connections can heat up and become a fire hazard.
Some conventional plug-outlet designs have twist-lock or screw-on covers that substantially prevent pulling force from withdrawing the plug. These are undesirable for vehicle connections, since if the vehicle moves, the plug and/or outlet are destroyed rather than simply unplugging as the boat or RV backs away from the docking station.
What is needed is an improved plug-outlet design that overcomes shortcomings of conventional outlet designs, while providing improved usability and/or safety characteristics.